System of multiple radio transmission



Sept. 20, 1932. w. HAHNEMANN SYSTEM OF MULTIPLE RADIO TRANSMISSION Filed Jan. 24, 1928 1,000,00 Loaqeag 000,000

Inventor n y M r. h H 0 H r WW 0 w d B Patented Sept. 20, .1932

UNITED STATES PATENT OFFICE WALTER HAHNEMANN, F BERLIN-MARIENFELDE, GERMANY, ASSIGNOR TO 0. LORENZ AKTIENGESELLSCHAFT, OF BERLIN, GERMANY SYSTEM OF MULTIPLE RADIO TRANSMISSION Application flied January 24, 1928, Serial No. 249,158, and in Germany February 8, 1927.

This invention has reference to a method of multiple radio transmissionwhich may be used for radio broadcasting or for transmission of distant signals.

An object of this invention is to prevent any local reduction of received signals due to the interference effect between two or more transmitting stations of the multiple transmission system.

0 Another object of this invention is to provide amethod for shifting the nodal lines of the interference pattern of the radiated signal at a rate above the audio frequency so that there will be no point where the node of 5 interference would constantly remain and reduce or eliminate the received energy.

A further object of this invention is to reduce the effect of fading of signals at a distant receiving station by causing the signals from the sending stations to change their relative phases or wave length at an inaudible rate, in such a way that the fading from the dif: ferent stations is not coincident in time.

-The invention is illustrated more clearly 5 in the appended figures as follows Fig. 1 is a plan view of the interference patterns of two stations which are operating at identical wave lengths and at coincident phase relation;

Fig. 2 is a diagrammatic illustration in elevation of the signals actually received in the space between the two stations illustrating further the reduction or the elimination of signal at the nodal points of interference, as applied to Fig. 1;

Fig. 3 in the interference pattern in plan view for the same two stations that are shown in Fig. 1, with the diflerence that the sta mitted signal of one or more stations according to the principle of this invention so as to produce the shifting of the interference pattern.

In Figs. 1 to 5 inclusive, reference charactersA' and B represent the location of two broadcasting stations. It has been suggested to use anumber of sending stations such as A and B located at suitable distances for the purpose of radio broadcasting, all of these stations operating on identical wave lengths and controlled by a common central station; this system has received the name of multiple broadcasting. The need for this system lies in the fact that the number of available wave lengths for broadcasting purposes is limited, and also, in the fact that the use of a plurality of small stations like A and distributed over the territory in question offers the best possibilities for good reception with simple receiving apparatus. This system is therefore preferable to the use of single large power sending stations. The first requirement for practical introduction of such multiple broadcasting telephony is the complete coincidence of all wave lengths of the transmitters in question so as to prevent the relative superposition and heterodyne disturbances of slightly differing wave lengths. A number of suggestions have been made that accomplish this practically.

A further phenomenon which becomes apparent with such a multiple broadcasting system is in the appearance of certain local eflfects, especially half way between two transmitters when the amount of energy is equal; in other cases this local effect is nearer to the less powerful sender. This effect consists in interference action of the oscillations radiated from different stations on each of the wave lengths of the frequency band used for transmission. This interference reduces more or less the energy at the receiving apparatus, causing more or less distortion of the telephonic modulation in such a receiver. With reference to the location of such dead places, that is, with disturbed reception, this would be in the form of a family of hyperbolae, shown by the full lines of Fig. 1 and Fig. 3 in the case of only appended Figures 2 and 4). It is the object of this invention to reduce or make practically harmless the above described disturbance and to make it possible to have clear reception at the above affected spots. The manner of this invention consists of supplying at the transmitter means for causing periodic interference changes in space or time in the district surrounding the sender, which changes occur so rapidly that no disturbing influence takes place at the receiver. This means that the frequency of these riodic changes must lie between the modulhiion frequency and the carrier frequency.

According to the invention this may be achieved for instance by a correspon periodic, suddenly or gradually repeated influencing of the phase of oscillations of the carrier frequency on one, several, or even all transmitters of the multiple system. In this case it is preferably that the amplitude of the base frequency is kept constant and that the frequency band radiated from the transmitter is not broadened too much. If the individual transmitters are located at a distance equal to a large number of wave lengths of the carrier frequency, then it is sured possible, according to this invention, to utilize corresponding periodic influencing of the carrier frequency itself (gradual, periodic and continuous phase shifting In Fig. 1, A and B represent two transmitters operating on identical wave lengths, the disizance from each othrlar as shown in this examp e 16 wave engths. Both of thesestationsoperateinthesamephaseso that the picture of the two instantaneous waves can be represented as in Fig. 1 by the full wavy line for station A and the broken wavy line for station B. -The interference pattern will therefore occur as illustrated where the lines represent the maximum interference and therefore the strength of reception.

The distribution of strength along the connecting line AB is then illus trated in Fig. 2. In this drawing, curves 0 and b show the attenuation of received energy from the two tters A and B respectively, when operating by themselves, as

summg of course that the two senders are of equal Reference character 0 represents the resulting receiving strength from both stations operating simultaneously, taking into account the above d interference action. scen,thc district most en is near the middle, between the two stations where the received energy is reduced almost to zero, while towards the stations the of each respective station relative to the other is gradually increasing so that the reduction 'ordistortion of modulation soon becomes negligible. v

InFig. 3 areillustrated twoimmsmitters to Fig. 1, except that the phase of transmitter B is shifted relatively to thephaseoftransmitterA. Inthiscasethc interference lines will be shifted so that now of reception will occur where previously were minima, and vice versa. is shown in Fig. 3 by the new interference pattern, the lines ofwhich will be seen to be shifted half way between the lines of pattern in Fig. 1.

Fig. 4 shows diagrammatically the distribution of signal strength in the space between the two stations. It can be seen to differ from Fig. 2 by having where Fig. 2 has and vice versa. According to J the invention, we may have for example a periodic changm' of the base of radiated wave of one or both statioiis (in the accompanying example this takes place on station B). The lines of maximum interference, i. e.. the stro reduction or disturbance is thereby subjected to periodic local shifting. In case these occur at a sufliciently high cadence not to affect the reception (that is, above the limit of the highest modulation frequency), then clear reception is ason a receiver at any location. The effect of this of interference pattern is illustrated more clearly in Fig. 5 where the two sets of lines are superimposed. The lines corresponding to Fig. 1 are shown full, those corresponding to Fig. 3 are shown broken. At any point in the broadcasting range itisimpossibletohaveaminimum ofsignal allthetime,sincewiththeshiftofthepattern the signal immediately changes to a maximumthus averaging a good strong reception. It is possible to achieve the described more or less sudden continuous changing of phases, for instance, by a rotating coil or condenser or else by any otheZlwell-known phase shifting arrangement. iron core choke coil would be especially suitable, the saturation of such a coil "may be changed by I alternating current which correspondsto any desired cadence of These choke coils can then influence the self-inductance or capacity in any desired manner, and any particular -ents, and thereby affect the phase of the circuit of the in questiorfl.

pen e change 0 recei of the additive oscillations at porn' t can ermore achieved according to thus mwentien by varying periodically the 1 frequency (and hence the wave length) is changed at each vertical line, representing time intervals of 1/20,000 second. The change in frequency may be only a very small percentage.

This is especially practical when the distance of the stations is large compared with the wavelength, since then the changes in wave length need not be very large. Other- 15 wise the wave length band for a definite'number of multiple broadcasting transmitting stations would be too large and the manner and advantage of the multiple broadcasting would be thereby set back or lost. The change of the carrier frequency may be secured very simply, for example, by subjectingthe tuning of one of the circuits that determines the frequency of the oscillations, such as the anode oscillating circuit of the tube sending arrangement, to a periodic change. Or the change of frequency could be secured for instance, by inclusion in one of the circuits of an iron core coil, the induction of which is changed by a. suitably controlled alternating current corresponding in frequency to the cadence at which the desired wave length chan e will occur. The same can be also achieve by inclusion of a variable condenser operated at a speed to create changes corresponding to the cadence of the desired wave length change.

A practical numerical example'may be submitted-as follows:

The distance of the most affected receiving location relative to the sender, that is, the most dangerous zone, is half the distance between the two stations. If a change in the strength of signal from 0 to about 1/2 is satisfactory at such a point, then it means that the phase at this point must be changed so that the interference lines (or spots) are shifted about 1/10 of the wave length. This would mean, in the present example, a wave length change at the sender of 1/10 X i from which can be easily derived, with a=%,

the number'of wave lengths (A) in the disof 300 meters corresponding to a frequency of 1,000,000 per second, it would be necessary to have a frequency change to 1,000,600 per second, and this change may take place at a cadence of 10,000 times per second. The required change of the interference positiongives therefore a value which in practice could not be designated as prohibitive or disturbing.

Such an eflfect as in the above described arrangements can be also achieved according to this invention by modulating the transmitted wave, for instance, at one of the stations, by an intermediate frequency, which is above the audible limit and is therefore not disturbing for reception. What happens then is that the local reception conditions represented by curve 0 according to Figs. 2 and 4 are changed periodically .in cadence with the modulating frequency into the normal continuously extended curves a or b. By this means the reception stren th is also changed, without a disturbing e ect, while the, same result as in the above is achieved. In this connection, when there are, e. g., three stations, it is only necessary to modulate one of these stations by the cadence frequency.

A further useful application of this invention may be described as follows It has been found that short waves (100 meters and less) are quite suitable for very distant transmission, even with a small amount of energy. But unfortunately the received signals are very often subjected to changes; commonly referred to as fading, which appear to be dependent on quite a number of different factors, suchas the time of day, the time of year, the wave length, atmospheric conditions, etc. A special instance of eccentricity of fading is the fact that during the daytime a certain wave length is best,.and at night another wave length is best. The facts of fading are sometimes explained as follows 5 In the case of a single wave length being transmitted from a single station to a single receiving station, the radiation may find two or more paths of differing lengths so that the waves arriving over the different routes will be out of phase. The plurality of different paths is then explained by the presence of re-- fleeting or refracting ionizedlayers (commonly called Heavyside layer).

There have been therefore, attempts to make the distant reception independent of these changes by simultaneous transmission on several different wave lengths, so as to maintain the legibility for a longer period of time.

According to the present invention, another way is suggested, which as distinct from the previous methods does not combat the eccentricities of fading by actually changing the wave length being transmitted or by simultaneously transmitting two wave lengths on the chance that one of the two will get through. Instead, a plurality of transmitting stations at different locations is employed, all using the same wave length. A plurality of receiving stations might be employed or both the sending and receiving stations might be pluralized.

But such short wave multiple transmission or reception without the inclusion of the present invention is impractical because the different lengths of the short wave routes would cause the waves to get out of phase and form nodal points. An interference pattern is therefore formed at the receiving location similar incertain respects to the one described above. While atv a certain distance there exists the addition of energy arriving along different routes, for another distance there is a subtraction. To attempt to move the station from such a point of signal would be futile because the various short wave routes are always varying in length due to the effects of light, darkness and the weather on the assumed Heavyside layer, and due to many other variations not yet understood. In other words, an interference pattern would be formed but it would be shifted fortuitously so as to accentuate rather than eliminate fading. The periods of shifting would probably be well within an audible cadence.

With the application of this invention to multiple short wave sending the above mentioned nodal or interference pattern would be varied at a cadence above the audible range and therefore without effect on modulation oscillations. Thus, all fading or other effects of variations in the lengt'h of the short wave paths due to changes in reflecting conditions or other conditions would be eliminated. According to this invention, a num-. ber of'spatially separated transmitters are provided which are all operating on the same wave length, and are controlled from a common central station. Although this latter arrangement is'disadvantageous on account of the increased cost, it may have exceptional v technical advantages in many cases in comparison to the large stations operating on long wave length with large energy output.

, If one of these sending stations is now .changed with regard to phase or carrier fre-- quency as described above, then the further eflt'ect of the changing interference pattern is introduced and the effect of the above described fading is' thereby minimized.

Having described my invention, what I believe to be new and desire tosecure and protect by Letters 1s I 1. In a system of multiple radio transmission comprising a number of transmitters operating on identical wave length and producing an interference pattern within the radiated range, a method of reducing the local Patent of the United States disturbing effects of interference which comprises periodically changing the phase on at least one of said transmitters and thereby shifting the interference pattern.

2. A method as specified in claim 1 in which said changing of phase has a cadence above the audible limit.

\ 3. In a system of multiple radio transmission comprising a number of spaced transmitters, normally operating on substantially identical wave lengths and producing an interference pattern within the radiated range, the method of reducing the effect of said intereference which comprises periodically shifting the interference pattern by a fraction of the wave length, at a cadence above the limit of audible frequency.

4. In a system of multiple radio transmission comprising a number of transmitters normally operating on substantially identical wave lengths and producing an interference pattern within the radiated range, the method of reducing the local disturbing effects of interference which comprises changing the carrier frequency of the radiation at one of said transmitters to produce a continual gradual change of phase of said transmitters and a resulting gradual shifting of said interference pattern.

5. A method as specifiedin claim 4 in which 7 said changing of carrier frequency has a cadence above the audible limit.

6. In a system of multiple transmission comprising a number of spaced transmitters 10 operating on substantially identical wave lengths and producing an interference pattern within the radiatingrange', the inethod of reducing the efiect of said interference which comprises periodically shifting the interference pattern by a fraction of the wave length at a cadence above the limits of the signalling frequency.

7 The method of eliminating fading which comprises operating a number of wave trans- 1m mitters of substantially identical wave lengths and periodically changing the phase of the radiated wave from one of said stations at 'a cadence above the limits of the signalling frequency.

8. The method of eliminating fading which comprises operating a number of wave transmltters of substantially identical wave lengths and periodically shifting the wave lengths of one of the stations by a fraction of a wave length ata cadence above the limits of the signalling frequency.

9. The method-of eliminating fading which comprises operating a number of wave transmitters of substantially the same frequency and changing the carrier frequency of one of these transmitters at a cadence above the limits of audibility. Y

10. In a system ofmultiple radio transmission comprising a number of transmitters 130 operating on substantially the same wave lengths and producing an interference pattern Within the radiated range, the method of reducing the local disturbing effect of interference which comprises changing the carrier frequency of the radiation at one of the transmitters at a predetermined frequency.

In testimony whereof I have afiixed my signature.

WALTER HAHNEMANN. 

